System for detecting conductive bodies



Jan. 7, 1958 w. c. HARMON 2,819,447

SYSTEM FOR DETECTING coNDUcTIvE BODIES Filed March 27, 1956 2Sheets-Sheet l IN VEN TOR. //l// /AM C. f/A /woN BYRUMI: s( MMU ATTORNEY"frans #Y Jan. 7, 1958 w. c. HARMON SYSTEM FOR DETECTING coNDUcTIvEBODIES Filed March 27, 1956 2 .Sheets-Shea*V n INVENTOR.

WML/AM C. HARA/10N I i l I I I I l l I l l l |lv|| ATTORNEY UnitedStates Patent O SYSTEM FOR DETECTING CONDUCTIVE BODIES Williani C.Harmon, Chagrin Falls, Ohio, assigner to Republic Steel Corporation,Cleveland, Ohio, a corporation of New Jersey Application March 27, 1956,Serial No. 574,312

18 Claims. (Cl. 324-41) The present invention relates to systems fordetecting conductive bodies in relatively nonconductive magnetic oreand, particularly, to such systems used to indicate the presence oftramp metal in a body of magnetic ore in movement from one point toanother.

Relatively large pieces of iron or other metallic substances, commonlyreferred to as tramp metal, may inadvertently be present in ore which itis desired to crush or pulverize by crushing machinery in certain -oreconcentration processes. Such tramp metal works down into the crusherwithout breaking up until it stalls the crusher vor breaks some part.This results in expensive repairs and lost production, and it isaccordingly desirable to detect all such tramp metal in the ore beforeit has an opportunity to reach the crusher.

The problem of consistently and reliably detecting tramp metal in orehas been an exceedingly troublesome one, .and many arrangements haveheretofore been proposed in an attempt to provide a satisfactorysolution. Magnets and magnetic rolls are in common use to remove straypieces of magnetic metal from nonmagnetic materials such as coal and thelike. =It has also been proposed that for the inspection of nonmagneticmaterials many forms of exploring inductors be so arranged that magnetictramp metal `shall induce a change in the magnetic flux of the inductorwhich change is then used to operate an alarm or control a system forrejecting the piece of tramp metal. None of the proposals of this typerespond to nonferrous or nonmagnetic material.

In an attempt to detect the presence of both nonmagnetic and magneticmetals, it has been proposed that a system of inductors be used in abridge arrangement. The inductors are carefully designed and sopositioned in balanced relation that there is normally substantiallyzero inductive coupling between an energized transmitter inductor and anenergizable receiver inductor, the inductive balance being upset by apiece of tramp metal in the magnetic field of Ithe inductors. Suchunbalance of inductive coupling is then indicated by an appropriatearrangement or is used to operate an alarm or automatic reject control.Excitation of the inductors is usually by a voltage of high audiofrequency, and relatively close balance of the inductor system must bemaintained to avoid false indications of the presence of tramp metal. Inpractice, the inductor balancing is found to be so critical that it isusually necessary to rebalance it about once every four hours duringoperation to counteract drift in spite of all precautions to insurestability.

ln an attempt to avoid the relatively critical inductive balance of thearrangement last described, it has been proposed that a tramp metaldetector employ a highfrequency oscillator in which an inductorconstitutes one element of a resonant circuit determining the frequencyof oscillation. This inductor is conventionally constructed with awindow through which the material under inspection may be moved, and anytramp metal present in the material then has the effect of changing theresistive and inductive impedance component ofthe inductor and therebyeffecting a change of oscillation amplitude or frequency or both. Thedetection of the tramp metal may according- 1y be based upon change ofoscillator frequency or change of oscillation amplitude or both changeof frequency and amplitude. When magnetic metal passes through thewindow of the exporing inductor, the magnetic properties of the metaltend to increase the amplitude of oscillation while eddy currentsinduced in the tramp metal tend to reduce the amplitude of oscillation.Accordingly there will theoretically be some critical size for a pieceof magnetic tramp metal where the two effects last mentioned balanceeach other and the piece of tramp metal fails to be detected. The sameresult occurs if the detection is premised upon change of the oscillatorfrequency. It has been found that if the frequency of oscillation ischosen sufficiently high, of the order of 50 kilocycles or higher, theeddy current losses induced in any solid piece of tramp metal whethermagnetic or nonmagnetic -outweigh any magnetic effect insofar as changesin amplitude of oscillation are concerned and thus always result in adecrease in the amplitude of oscillation. Operation on these highfrequencies, however, requires that all energizing voltages supplied tothe oscillatory system be carefully regulated in amplitude and thatspecial precautions be taken to make the equipment relatively immune tochanges in system parameters with changes in temperature and humidity.In spite of all such precautions, it has been found in practice that theoscillator amplitude changes slowly over a period of time and thisrequires resetting of one or more oscillator controls about once everytwo hours if reasonably reliable and consistent operation is to beeffected.

A serious disadvantage and limitation inherent in the oscillatory systemlast described, particularly in the case of tramp metal detection inmagnetic ores such as taconite, is that the finely divided iron oxideparticles of the ore create little eddy current loss t-o reduce theimpedance of the exploring inductor whereas the magnetic properties ofthe ore effect substantial increase of the inductance and impedance ofthe inductor. This results in increasing the amplitude of oscillationalmost in a direct ratio to the incremental volume of ore passing-through the window of the exploring inductor, and this effect must becounterbalanced by a sufficiently high operating frequency that theinduced eddy current losses in a piece of tramp metal (whether magneticor nonmagnetic) shall always decrease the amplitude of oscillationsuicient to override the magnetic effect of the iron ore. The oreconventionally is moved on a rubber conveyor belt through the Window ofthe exploring inductor, and if the incremental volume or quantity of oremoving through the inductor was always uniformly the same it would befeasible to adjust the oscillator for operation at a convenientamplitude level with this quantity of ore in the exploring inductor.This is not readily feasible in practice, however, for the reason thatthe ore on the conveyor belt varies both in quantity and quality (thepercentage of iron in the ore). Thus if the oscillator is adjusted tosome average value of oscillation amplitude and the quantity or qualityof the ore then decreases below the average, the amplitude ofoscillation also decreases and provides an indication of the presence ofa piece of tramp metal in the ore although no tramp metal is present infact. Conversely, if the quantity and quality of the ore passing throughthe exploring inductor increases, the amplitude of oscillation increasesabove the average and a small or medium size piece of tramp metal maynot reduce the amplitude of oscillation to a value below the average andthus fails to be detected.

It is an object of the present invention to provide a new and improvedoscillatory system for use in detecting with high sensitivity conductivebodies present in a moving body of relatively nonconductive magneticore, and i one which avoids one or more of the disadvantages andllimitations of prior proposed tramp metal detecting sysems.

It is a further object of the invention to provide a novel oscillatory.system characterized by high and cons1stently reliable transientresponse to moving conductive bodies of a wide range of sizes and ofboth magnetic and nonmagnetic materials yet one providing relativelylittle response, either transient or of longer duration, to a movingbody of magnetic ore having widely varying magnetic values by virtue ofeither the quality or quantity of ore.

It is an additional object of the invention to provide an improvedoscillatory system which is relatively free of undesired false responseto reactive electrical properties of varying quantities and qualities ofmoving magnetic ore as well as to long-term changes in system operatingparameters while retaining optimum desired transient response toresistive electrical properties of moving conductive bodies even ofrelatively small size.

It is a further object of the invention to provide a unique oscillationgenerator of the type having a resonant circuit determining thefrequency of oscillation, and including an inductor with a windowthrough which a low of magnetic ore moves, and one in which the reactivecomponent of impedance of the inductor primarily controls the operatingfrequency but has insigniticant control over the amplitude ofoscillation and may vary appreciably with changing quantities andqualities of moving ore whereas the resistive component of impedance ofthe inductor primarily controls transient oscillation amplitude changesof the system thus to enable a high and stabilized sensitivity of thesystem to the detection of conductive bodies included in the moving ore.

Other objects and advantages of the invention will appear .as thedetailed description thereof proceeds in the light of the drawingsforming a part of this application and in which:

Fig. 1 illustrates in elevational view, and Fig. 2 in cross-sectionalView, the construction of an exploring inductor suitable for use in anoscillatory system embodying the present invention; and

Fig. 3 represents, partly schematically, a tramp metal detecting systemembodying an oscillatory system of the present invention in a particularform.

Referring more particularly to Figs. l and 2, the exploring inductorassembly comprises a rectangular frame 11 which may be of wood orplastic or the like material having a circumferential groove 12 in whicha number of turns of wire are placed to form an inductor 13. The frame11 with its inductor 13 is xedly supported within the window of a hollowframe or housing 14 which may be constructed of wood or like materialand which has a rectangular doughnut configuration of rectangular crosssection as shown. This configuration i-s merely a convenient one whichmay be used to support a suitable electromagnetic shield 15 in spacedrelation to and surrounding the inductor 13. The shield 15 mayconveniently be constructed of small mesh hardware cloth, folded aboutthe housing 14 while preserving the open window of the latter, havingall fold laps electrically connected vas by the use of solder to providea continuous electrically conductive shield surface entirely coveringthe exterior surface of the housing 14 except for its window. Thehousing construction preferably employs nonconductive fasteners such aswood dowel pins to hold the assembly together although brass screws andbrass washers may be used in limited numbers to hold the shield 15 inplace on the housing.

By way of example of suitable parameter-s for a representative exploringinductor, the frame 12 may have a height of l2 inches with a width of3:8 inches ,and the groove 12 may be -/s inch deep and 3A; inch wide.The inductor 13 may be wound with 2O turns yof #29 enamel covered wire.The shield 15 may be comprised of l/g inch mesh hardware cloth, and thedimensions of the housing 14 .are so selected that the shield 15 isspaced not closer than 8 inches from the inductor 13 at all points.

The exploring inductor and its shield housing are fixedly supported bymeans not shown in such relation to an ore conveyor belt 16 that thelatter extends through the window of the exploring inductor and shieldhousing. Ilt will be understood that the conveyor belt is supported inconventional manner upon support rollers, not shown, and is suitablydriven to move -a body of ore 17 at convenient linear velocity throughthe window of the exploring inductor.

The exploring inductor above described is included as one component of aresonant tank circuit of an oscillatory system presently to be describedand conveniently positioned a short distance away. The inductor isconnected .to this oscillatory system through a shielded flexiblecoaxialcable 18.

The circuit arrangement of the oscillatory system last mention is shownin Fig. 3 as enclosed within a broken line rectangle 19, and includes atriode form of vacuum tube 20 having a control electrode 21 and cathode22 coupled to the inductor 13 through a grid-current limiting resistor23 and a coupling condenser 24. A condenser 25 is connected in shunt tothe inductor 13 to constitute with the latter a resonant circuit havinga resonant frequency of approximately 86,000 cycles per second. Thevacuum tube 20 includes a conventional cathode-circuit self-biasingnetwork, comprised by a resistor 26 and parallel-connected condenser 27,which biases the tube to the linear portion of its operatingcharacteristic and the control electrode 21 of this tube is connected toground through the resistor 23 and a grid-leak resistor 28. The tube 20includes an anode 29 which is coupled through an anode load resistor 30and a decoupling resistor 31 to a source of anode potential, indicatedas +B.

The oscillatory system 19 also includes a second triode form of vacuumtube 33 having a control electrode 34 and a cathode 35 coupled to theoutput circuit of the vacuum tube 20 through a condenser 36 and apotential divider which, for energy of oscillatory frequency, iscomprised by a resistor 37, a potentiometer 38, a resistor 39 and acondenser 40. A movable contact 41 of the potentiometer 38 is coupledthrough a grid-current limiting resistor 42 to the control electrode 34of the tube 33, and the latter includes a cathode-circuit self-biasnetwork comprised by a resistor 43 and shunt connected condenser 44which biases this tube to the linear portion of its operatingcharacteristic. The tube 33 includes .an anode 45 which is coupled tothe anode potential source +B through a load resistor 46 and thedecoupling resistor 31, the latter being one component of a decouplingnetwork of conventional arrangement including a filter condenser 47which connects the low potential terminal of the resistor 31 to ground.A degenerative energy-feed back resistor 48 couples the anode 45 of thetube 33 and the anode 29 of the tube 20 to improve in Well known mannerthe stability and linearity of the operating characteristics of thetubes 20 and 33 which, with their associated circuit components,essentially comprise a tandem arranged two stage resistance coupledamplifier. To cause the system just described to generate sustainedoscillations, the output circuit of the vacuum tube 33 is coupledthrough an adjustable condenser 50 to the input circuit ofthe vacuumtube 20.

The oscillatory system 19 is included in a tramp metal detector which,as shown -in Fig. 3, includes a conventional high-frequency broad-bandamplifier 52 coupled through a condenser 53 and the condenser 36 totheoutput circuit of the vacuum tube 20, an amplitude demodulator 54coupled to the output circuit of the a1nplit`1er.52, and a conventionallow-frequency amplier and relay control system 55 which is coupled tothe output circuit of the demodulator 54 and 4includes in its outputcircuit a suitable relay 56 having contacts 57 used to control asuitable tramp metal alarm or tramp metal reject system not shown. Thebroad-band amplifier 52 has a broad pass-band characteristicapproximately centered about the nominal oscillatory frequency (86kilocycles per second) of the oscillatory system 19, and to this end maybe comprised by a single-stage resistance coupled amplifier withbroad-band translation characteristic. The demodulator 54 includes atriode form of vacuum tube 58 operated as a diode rectifier by havingits control electrode 59 and anode 60 connected together and coupledthrough a condenser 61 to the output circuit of the amplifier 52. Thecathode 62 of the tube 60 is connected to ground, and the demodulatorincludes a load resistor comprising a fixed resistor 63 in series with apotentiometer 64. The movable contact 65 of the potentiometer is coupledthrough an adjustable resistor 66 and a resistor 67, comprising elementsof a filter network which includes the condenser 40, to the junction ofthe voltage divider resistor 39 and bypass condenser 40. A filterresistor 68 couples the demodulator 54 to the low-frequency ampliiierand relay control unit 55, which is of conventional arrangement and mayhave the general construction shown in the Harmon et al. United StatesPatent 2,660,704.

Considering now in greater detail the operation of the oscillatorysystem 19, oscillations appearing in the resonant circuit comprised bythe inductor 13 and condenser 25 are amplified by the vacuum tubes 20and 33 and the amplified oscillations are fed back through the feed-backcondenser Si! in phase with the oscillations appearing in the inputcircuit of the tube 20. The amplitude of the amplified oscillations thusfed back through the condenser Sti to the input circuit of the tube 20are so controlled by adjustment of the contact 41 of the potentiometer38 as to maintain the system in oscillation at a satisfactory level butnot at a sufficiently high level as to overdrive the system and cause anunstable oscillatory condition.

The resultant oscillations after amplification in the amplifier 52 arerectified by the demodulator 54 to derive across the potentiometer 64 anegative gain control unidirectional potential having an amplitudevarying with the amplitude of oscillation of the oscillatory system 19.A portion of this gain control potential, as determined by manualadjustment of the position of the potentiometer contact 65, is appliedthrough the filter network comprised by the resistors 66 and 67 and thecondenser 40 to the control electrode 34 of the tube 33 for purposes ofcontrolling the transconductance and thereby the gain of this tube. Thetime constant of the filter network 40, 66 and 67 is so selected byadjustment of the resistor 66 that the gain control potential incontrolling the gain f the tube 33 tends to maintain the steady-stateamplitude of oscillation substantially constant for all exceptrelatively short transient amplitude changes. This feature makes thecircuit relatively immune to drift and largely eliminates any need forresetting or adjustment after the instrument is installed. This is aunique feature. Conveyor belt speeds ordinarily range from perhaps 100feet per minute` to 400 feet per minute. For lower belt speeds, it isdesirable to increase the time constant of the network 66, 67 and i0 byincreasing the Value of the resistor 66. Otherwise the compensatingaction of the gain control circuit may be fast enough to level out thechange of amplitude of oscillation as a piece of tramp metal passesthrough the exploring inductor. Conversely, higher belt speeds make itdesirable to decrease the time constant of the filter time-constantnetwork to permit the oscillator to recover normal amplitude morerapidly after change of its oscillation amplitude produced by a piece oftramp metal passing through the exploring inductor. Quick recoveryenables the system to indicate the presence of a second piece of trampmetal following closely behind the irs't. While belt speeds of 100 feetper minute and 6 400 feet per minute are cited as being the limitsof arepresentative range, it will be understood that these values are merelyrepresentative and that the upper and lower belt speeds are notcritical.

In addition to the oscillation amplitude control provided by adjustmentof the potentiometer 38 and the action of the gain control potential aslast mentioned, it will be apparent that the feed back condenser 50 hasa reactive impedance varying with the value of capacitance of thiscondenser so that manual adjustment of its capacitance may also vary theoscillation amplitude by increasing or decreasing the magnitude of thefeed-back energy. Furthermore, the reactance of the condenser 50 variesinversely with the frequency of oscillation of the oscillatory system sothat increasing frequency effects an increase in the magnitude of thefeed-back energy and vice versa. The importance of this will becomeapparent upon consideration of the effect of varying quantities andqualities of magnetic ore moving on the conveyor belt through the windowof the exploring inductor 13. The finely divided iron oxide in the oreprovides additional paths for the magnetic flux around the inductor 13so that the inductance, and thereby the impedance, of the inductorincreases with the quantity (and also the quality) of ore moving throughthe inductor. Any such increase in impedance will produce acorresponding increase in the amplitude of oscillations because a givenvalue of feed-bach energy through the condenser 50 is enabled to developa larger oscillatory voltage in the input circuit of the tube Zt andthis in turn causes a corresponding increase in the amplitude of thefeed-back energy. However, the same effect which increases theinductance and impedance of the inductor 13 also lowers the frequencyo-f the generated oscillations because the frequency of oscillation ofthe system is determined by the combined inductance of the inductor 13and the fixed value of capacitance of the condenser 25. Thus two effectstake place in direct ratio to the increasing amount and quality ofmagnetic ore moving through the window of the inductor 13; first theamplitude of the generated oscillations is increased, and secondly thefrequency of the oscillations is decreased.

The foregoing description of the oscillatory system operation neglectsthe effect of the variable feed back condenser 50. If the capacitance ofthe condenser 50 is made quite large, as for example of the order of30() micromicrofarads, the system will operate as above described. Onthe other hand, if the capacitance of the condenser 50 is of smallervalue (perhaps of the order of rnicromicrofarads) a condition can beobtained where the voltage drop across the reactive impedance of thecondenser 50 increases with decreasing oscillation frequency in the sameratio as the impedance of the inductor 13 increases. In this case, anytendency of the oscillatory voltage across the tuned circuit 13, 2S toincrease due to increase of impedance of the inductor 13 by a largerquantity or higher quality of magnetic ore passing through the window ofthe latter is counteracted or neutralized by a reduction in the amountof energy fed back through the condenser 50 due to the higher voltagedrop across the latter caused by the lower oscillatory frequencyresulting from the increased value of inductance of the inductor 13.Therefore, by appropriate choice or adjustment of the value ofcapacitance of the feed-back condenser 50, a stabilized amplitudecharacteristic can be established for the oscillatory system 19 suchthat the amplitude of oscillation does not change even though thequantity of ore moving through the window of the inductor 13 varies fromzero to the full extent that can pass through the window. This value ofthe condenser 50 provides tight amplitude stabilization for varyingquantities of ore, and is readily established by adjustment in practicesince too large a value will result in an increase of oscillationamplitude with 7 increasing quantities of ore moving through theinductor 13 Whereas too small a capacity will result in a decrease ofoscillation amplitude with increasing quantities of ore moving throughthe inductor.

In initially adjusting the oscillatory system operation to attain theamplitude-stabilized characteristic last dcscribed, the potentiometer 38is adjusted concurrently with adjustment of the value of capacitance ofthe condenser 50. As pointed out above, the oscillatory system 19 isoperated at a standard oscillation level such that the amplifier tubes2t) and 33 essentially operate as linear amplifiers and at anoscillation level sufficiently low that it does not cause either of thecontrol electrodes 21 and 34 to draw any appreciable control-electrodecurrent by peak rectification of the oscillations applied to the controlelectrodes. To this end, each time that the feed-back condenser 50 isadjusted to a different value of capacitance the contact 41 of thepotentiometer 38 is also adjusted to restore the oscillatory system 19to its predetermined or standard oscillation level. The desiredoperating condition of the system, effected by adjustment of thecondenser 50 and potentiometer 38, is the one mentioned above where anyquantity of ore from Zero to maximum may be moved through the window ofthe exploring inductor 13 without appreciably changing the amplitudelevel of oscillations generated by the oscillatory system 19. Tofacilitate adjustment of the condenser 50, it is preferably providedwith a large calibrated dial and a lock for locking the condenser inadjusted position.

With the system adjusted for optimum operation in the manner justdescribed, any metallic body whether magnetic or nonmagnetic uponentering7 the field of the exploring inductor 13 has high frequency eddycurrents induced in it. These induced eddy currents cause the resistivecomponent of impedance of the exploring inductor 13 to increase. Thiscauses the oscillatory circuit 13, 25 to be more heavily loaded andthereby causes the amplitude of oscillation to decrease (in severe casesthe oscillatory system may cease to oscillate entirely). It will beappreciated that this change of oscillation amplitude is of transitorynature, occurring only during the interval the conductive metal moveswithin the highfrequency field of the inductor 13. The time constant ofthe gain control system filter network 40, 66 and 67 is selectedsufficiently long that the gain control system is not effective tochange the gain of the tube 33 rapidly enough to counteract to anyappreciable extent any such transient amplitude change as lastdescribed.

The amplifier 52 amplilies these transient amplitude changes, and thedemodulator 54 peak rectifies the amplified oscillations to derive atransient voltage across its load resistors 63 and 64. This transientvoltage is applied to one or more low-frequency amplifier stages of theunit 55, which may include one or more integrating networks to sharpenthe transient nature of the derived voltage, and the amplified transientvoltage may be used to trigger one or more tandem arranged gas dischargedevices the last of which controls the operation of the relay 56.

Thus by amplifying the output of the oscillatory system 19 to detectsmall transient decreases in oscillation amplitude, very small pieces oftramp metal may be detected. Since, as pointed out above, the quantityand quality of ore moving through the window `of the inductor 13 havesubstantially no effect on the oscillation amplitude, such small piecesof tramp metal may be detected regardless of whether the window of theinductor 13 is empty, partially filled, or full of moving ore. It hasbeen found in practice that this is true even though the ore issaturated with water. A representative sensitivity setting of the systemmay be made such that the system indicates the presence of a badly wornnonmagnetic steel dipper tooth having approximate dimensions of fourinches wide, ve inches high, and three inches deep. With this setting,the presence of much smaller pieces `of magnetic steel of theapproximate order of one-quarter of the mass of the nonmagnetic dippertooth will readily be indicated. This order of sensitivity has beenfound in practice to provide operation free from false indications frompieces of conductive `ore while at the same time providing sufficientsensitivity to detect pieces of tramp metal large enough to damage theore Crusher.

The component values shown in the drawing are included merely asrepresenting values which had been found satisfactory in practice andare not intended to limit the invention to any particular designconstants.

lt will be apparent from the foregoing description of the invention thatan oscillatory system embodying the invention exhibits high transientresponse to moving conductive bodies of a wide range of sizes while yetexhibiting relatively little or no response ether transient or other-Wise to a moving body of magnetic ore of widely varying quantity andquality. Thus the oscillatory system of the invention is relatively freeof undesired false response to reactive electrical properties of varyingquantities and qualities of moving magnetic ore as well as to long termchanges in system operating parameters while yet retaining optimumdesired transient response to resistive electrical properties of movingconductive bodies even of relatively small size. It will be evident thatthese novel operating characteristics directly result from the abilityof the system to use the reactive component of impedance of theexploring inductor primarily to control the operating frequency, but tohave substantially no effect on the oscillation amplitude, Whereas theresistive component of impedance of the exploring inductor primarilycontrols the transient oscillation amplitude changes and thereby enablesthe system to have high and stabilized sensitivity for detection ofconductive bodies either of magnetic or nonmagnetic metal.

While a specific form of the invention has been described for purposesof illustration, it is contemplated that numerous changes may be madewithout departing from the spirit of the invention.

What is claimed is:

l. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aow of ore may move, means including said resonant circuit for generatingelectrical oscillations of frequency controlled by said circuit, andfrequency-responsive oscillation-sustaining means for rendering theamplitude of said generated oscillations substantially constant withchanges of the reactive impedance of said inductor caused by changingvolumes of magnetic ore passing through said window.

2. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aow of ore may move, means including said resonant circuit for generatingelectrical oscillations of frequency controlled by said circuit, andfrequency-responsive oscillation-sustaining mcans for rendering theamplitude of said generated oscillations substantially constant withchanges of the reactive impedance of said inductor caused by changingvolumes of magnetic ore passing through said window while permittingtran lent amplitude changes of said generated oscillations caused bytransient change in the resistive impedance of said inductor due topassage of conductive bodies through the window thereof.

3. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aow of ore may move, means including said resonant circuit for generatingelectrical oscillations of frequency controlled by said circuit, saidmeans including an oscillatory-energy feed-back path for at leastpartially sustaining the generation of said oscillations, and animpedance having an impedance component varying with the frequency ofgenerated oscillations to control the magnitude of energy fed back bysaid path inversely with changes of impedance of said inductor caused bychanging magnetic properties of magnetic ore moving through said window.

4. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which allow of ore may move, means including said resonant circuit forgenerating electrical oscillations of frequency controlled by saidcircuit, and oscillation-sustaining means including impedance meanshaving an impedance component varying with the frequency of saidgenerated oscillations for rendering the amplitude of said generatedoscillations substantially constant with changes of the reactiveimpedance of said inductor caused by changing volumes of magnetic orepassing through said window.

5. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore cornprising, a resonant electricalcircuit including an exploring inductor having a window through which aflow of ore may move, means including said resonant circuit foramplifying electrical oscillations generated in said circuit, and anoscillation-sustaining feed-back path including an impedance elementhaving an impedance component varying with the frequency of saidgenerated oscillations for rendering the amplitude of said generatedoscillations substantially constant with changes of the reactiveimpedance of said inductor caused by changing volumes of magnetic orepassing though said window.

6. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aow of ore may move, means including said resonant circuit for amplifyingelectrical oscillations generated in said circuit, and anoscillation-sustaining energy-feed-back condenser having a valueselected to render the amplitude of said generated oscillationssubstantially constant with changes of the reactive impedance of saidinductor caused by changing volumes of magnetic ore passing through saidwindow.

7. An oscillatory system adapted to detect conductive bodies inrelatively non-conductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aow of ore may move, reactive means having a reactive impedance varyingwith frequency, and oscillation-sustaining energy-translating meansincluding said resonant circuit and said reactive means for generatingsustained oscillations having substantially constant amplitude withchanges of the reactive impedance of said inductor caused by changingvolumes of magnetic ore passing through said window.

8. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aflow of ore may move, means including said resonant circuit forgenerating electrical oscillations of frequency controlled by saidcircuit, frequency-responsive oscillation sustaining means for exertinga control over the amplitude of generated oscillations inversely Withchanges of the reactive impedance of said inductor caused by changingvolumes of magnetic ore passing through said window, and means includingan amplitude demodulator for maintaining a substantially linearinput-output oscillation translation characteristic in said oscillationgenerating means for oscillation amplitude changes other than thosetransient Aamplitude changes due to change in the resistive imped- 10ance of said inductor bythe passage of conductive bodies through saidwindow.

9. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aiiow of ore may move, oscillatory energy amplifying means having aninput circuit including said resonant circuit and an output circuit inwhich amplified oscillatory energy is developed and exhibiting asubstantially linear input-output amplitude translation characteristicfor any amplitude of input-circuit oscillations below a predeterminedvalue and means having an oscillation translation characteristic varyingwith frequency for translating oscillatory energy from said outputcircuit to said input circuit to generate sustained oscillations havinginput-circuit amplitude less than said predetermined value and remainingsubstantially constant with changes of the reactive impedance of saidinductor caused by changing volumes of magnetic ore passing through saidwindow.

l0. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore cornprising, a resonant electricalcircuit including an exploring inductor having a window through which aflow of ore may move, oscillatory-energy amplifying means having aninput circuit including said resonant circuit and an output circuit inwhich amplified oscillatory energy is developed and exhibiting asubstantially linear input-output amplitude translation characteristicfor any amplitude of input-circuit oscillations below a predeterminedvalue, a first translation means having an oscillation translationcharacteristic substantially independent of oscilla,- tion frequency foradjustably controlling the amplification ratio of said amplifying means,and a second translation means having an oscillation translationcharacteristic varying with frequency for translating oscillatory energyfrom said output circuit to generate sustained oscillations having undercontrol of said first translating means an input-circuit amplitude lessthan said predetermined value, the related magnitudes of oscillatoryenergy translated by said rst and second translating means beingselected to maintain the input-circuit amplitude of said generatedoscillations substantially constant with changes of the reactiveimpedance of said inductor caused by changing volumes of magnetic orepassing through said window.

1l. An oscillatory system adapted to detect conductive bodies inrelatively noncondu-ctive magnetic -ore comprising, Va resonantelectrical circuit including an exploring inductor having a windowthrough which a ow of ore may move, oscillatory energy amplifying meanshaving an input circuit including said resonant circuit and an outputcircuit in which amplified oscillatory energy is developed andexhibiting a substantially linear inputoutput amplitude translationcharacteristic for any amplitude of input-circuit oscillations below apredetermined value, feed-back means having an oscillation translationcharacteristic varying with frequency for translating said oscillatoryenergy from said output circuit to said input circuit to generatesustained oscillations, and a relatively longtime-constant automaticgain control system coupled to said output circuit and responsive tosaid generated oscillations for controlling the gain of said amplifyingmeans to maintain the steady-state input-circuit amplitude of saidoscillations less than said predetermined value, the value of energy fedback by said feed-back means being selected to render the steady-stateoscillation amplitude substantially constant with changes of thereactive impedance of said inductor caused by changing volumes ofmagnetic ore passing through said window.

12. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which ailow of ore may move, oscillatory energy amplifying means having aninput circuit including said resonant circuit and an output circuit inwhich amplied oscillatory energy is developed, means having anoscillation translation characteristic varying with frequency fortranslating oscillatory energy from said output circuit to said inputcircuit to generate sustained oscillations, and a gain control systemfor controlling the gain of said amplifying means to maintain asteady-state linear input-output amplitude translation characteristicthereof and including a relatively long-time-constant impedance networkestablishing a limit for the maximum duration of detectable transientchanges of amplitude of the generated oscillations, the steady-statevalue of said feed-back energy being selected to maintain the amplitudeof said generated oscillations substantially constant with changes ofthe reactive irnpedance of said inductor caused by changing volumes ofmagnetic ore passing through said window but to permit transientamplitude changes in response to conductive bodies passing through saidwindow.

13. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aflow of ore may move, a multi-stage tandem arranged resistance coupledamplifier having an input circuit including said resonant circuit and anoutput circuit in which amplified oscillatory energy is developed andexhibiting a substantially linear input-output amplitude translationcharacteristic for any amplitude of input-circuit oscillations below apredetermined value and a substantially constant inputoutput amplitudetranslation characteristic over a predetermined frequency band, andmeans having an oscillation translation characteristic varying withfrequency for translating oscillatory energy from said output circuit tosaid input circuit to generate sustained oscillations within saidfrequency band and of an input-circuit amplitude less than saidpredetermined value and remaining substantially constant with changes ofreactive impedance of said inductor caused by changing volumes ofmagnetic ore passing through said window.

14. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aflow of ore may move, reactive means having a reactive impedance varyingwith frequency, oscillation amplifying means including a gain controlsystem for maintaining the operation of said amplifying means within asubstantially linear input-output translation characteristic, and anoscillation-sustaining energy feed-back translating path including saidresonant circuit and said reactive means for feeding oscillatory energyfrom the output circuit to the input circuit of said amplifying means togenerate sustained oscillations having substantially constant amplitudewith changes of the reactive impedance of said inductor caused byvarying magnetic properties of magnetic ore while permitting transientamplitude changes with changes of the resistive impedance of saidinductor caused by conductive bodies passing through said window.

15. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an eXploring inductor having a window through which aflow of ore may move, means including said resonant circuit forgenerating electrical oscillations of frequency controlled by saidcircuit, oscillation amplifying means including a gain control systemfor maintaining the operation of said amplifying means within asubstantially linear inputoutput translation characteristic, and anoscillatory-energy feed-back path including a condenser and saidresonant circuit in series in said path for coupling oscillatory energyfrom the output circuit to the input circuit of said `amplifying meansto generate sustained oscillations of substantially constant amplitudewith changes of the reactive impedance of said inductor caused bychanging volumes of magnetic ore passing through said window whilepermitting transient amplitude changes with the resistive impedance ofsaid inductor caused by conductive bodies passing through said window.

16. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aflow of ore may move, means includingsaid resonant circuit forgenerating electrical oscillations of frequency controlled by saidcircuit, frequency-responsive oscillation-sustaining means for renderingthe amplitude of said generating oscillations substantially constantwith change of the reactive impedance of said inductor caused bychanging volurnes of magnetic ore passing through said window, and meansfor detecting transient amplitude changes of said generated oscillationswith transient change in the resistive impedance of said inductor duc topassage of conductive bodies through the window thereof.

i7. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aflow of ore may move, means including said resonant circuit forgenerating electrical oscillations of frequency controlled by saidcircuit, an oscillation-sustaining energy-feed-back condenser having avalue selected to render the amplitude of said generated oscillationssubstantially constant with changes of the reactive impedance of saidinductor caused by changing Volumes of magnetic ore passing through saidwindow, and a peak rectifier responsive to said generated oscillationsfor detecting transient amplitude changes thereof with transient changein the resistive impedance of said inductor due to passage of conductivebodies through ysaid window.

18. An oscillatory system adapted to detect conductive bodies inrelatively nonconductive magnetic ore comprising, a resonant electricalcircuit including an exploring inductor having a window through which aHow of ore may move, means including said resonant circuit forgenerating electrical oscillations of frequency controlled by saidcircuit, frequency-responsive oscillation-sustaining means for renderingthe amplitude of said generated oscillations substantially constant withchanges of the reactive impedance of said inductor caused by changingvolumes of magnetic ore passing through said window, means for detectingtransient amplitude changes of said generated oscillations withtransient change in the resistive imped ance of said inductor due topassage of conductive bodies through the window thereof, and meansresponsive to said detected transient amplitude changes for providing acontrol effect indicating the presence of said conductive body at saidwindow.

References Cited in the tile of this patent UNITED STATES PATENTS2,267,884 Zuschlag Dec. 30, 1941 2,489,920 Michel Nov. 29, 19492,580,670 Gilbert Ian. l, 1952 2,744,232 Shawhan. et al. May l, 19562,753,520 Doll July 3, 1956

